1. Field of the Invention
The present invention relates to a platinum group impurity recovery liquid and a method for recovering a platinum group impurity, which can effectively recover a platinum group impurity among contaminants on a surface of a silicon substrate or in a film thereon.
2. Description of the Related Art
Recently, in the electronic industry, as electronic devices have been miniaturized and the integration density thereof has been increased, it has become significantly important to reduce contamination of such devices resulting from materials and production processes used. Since heavy metal contamination particularly degrades the performance of a semiconductor device remarkably, it must be made every effort to eliminate such contamination. In order to detect such problematic metal contamination, a super sensitive metal impurity analysis technique is required.
In general, as metal impurity analysis techniques, chemical analyses are often used due to their high detectability. An impurity metal on the surface of a substrate is generally recovered using a vapor phase decomposition (VPD) method. More specifically, when a hydrofluoric acid gas, or the like, reacts with an oxide film on the silicon substrate, the oxide film is decomposed, thereby producing a decomposed liquid on the surface of the substrate. A recovery liquid is dripped onto the surface of the substrate, and the droplets are, for example, rolled, so that the decomposed liquid on the surface of the silicon substrate is recovered. Then, the obtained solution in which the impurity metal is dissolved is quantitatively measured using an atomic absorption spectrometry (AAS) analyzer or an inductively coupled plasma-mass spectrometry (ICP-MS) analyzer.
With chemical analyses, which are conventionally used for analyzing a metal impurity on the surface of a silicon substrate or in a thin film thereon, it is difficult to recover a noble metal element having an ionization tendency smaller than that of silicon, particularly, Pt. This is because noble metal elements cannot be sufficiently decomposed by a hydrofluoric acid vapor, and even if the noble metal elements could be decomposed, they are reattached to the surface of the silicon substrate due to their ionization tendencies smaller than that of silicon. As a method for recovering Pt, there exists a technique for performing a recovery operation using aqua regia which can dissolve a noble metal as a recovery liquid (Institute of Electronics, Information and Communication Engineers SDM91-159).
In the VPD method, however, since there is a limit in the amount of a recovery liquid which can be used, the amount of an aqua regia recovery liquid which sufficiently can dissolve Pt cannot be used. Therefore, a high recovery rate cannot be obtained by one time recovery operation. A recovery rate of Pt which can be obtained by one time recovery operation is about 70%, and in order to obtain a recovery rate of 90% or more, the recovery operation needs to be repeated twice or more. Moreover, among the noble metal elements, Pt can be dissolved only in heated aqua regia. Therefore, an aqua regia recovery operation needs to be performed while heating a substrate, or using heated aqua regia. Thus, if aqua regia is dripped onto a slightly overheated substrate, aqua regia violently splashes around the substrate, resulting in poor quantitativeness and safety as an analysis method. Moreover, if only a small portion of aqua regia is evaporated or splashed by heating aqua regia on the substrate, it is difficult to maintain the amount of the recovery liquid, and thus the result of the quantitative analysis is significantly affected. Furthermore, it is necessary to provide a local ventilation so that acidic atmosphere is not inhaled during the operation, and it is required to operate in a clean draft chamber whose environment is much safer than an ordinary environment.
In order to solve these problems, Japanese Patent No. 2701813 discloses a method for analyzing an impurity on a surface of a silicon substrate or in a thin film thereon by using, as a recovery liquid in the VPD method, any one of chlorine water, a mixed liquid of chlorine water and ozone water, and electrolytic ion water in which hydrochloric acid is employed as a supporting salt (Conventional Example 1). In Conventional Example 1, it is possible to perform the VPD process without heating the recovery liquid or the silicon substrate, thereby achieving certain measure of safety in the analysis operation.
Japanese Patent Laid-Open Publication No. Hei.7-193035 discloses a method for recovering an impurity element on a surface of a silicon wafer using a solution composed of hydrofluoric acid, hydrogen peroxide, and water, or a solution composed of hydrofluoric acid, hydrogen peroxide, hydrochloric acid, and water as a recovery liquid for recovering an impurity such as Cu whose ionization tendency is smaller than that of silicon (Conventional Example 2). In Conventional Example 2, a Cu-contaminated wafer is produced by contaminating a. surface of a wafer with Cu. As a solution for recovering a contaminant, hydrofluoric acid, hydrogen peroxide, hydrochloric acid, and ultrapure water are mixed so that each of the concentrations of the components of the solution is adjusted to be 5% by weight. Thereafter, the recovery liquid is dripped onto the Cu-contaminated wafer so as to scan the entire surface of the wafer twice in five minutes, and then the recovery liquid is recovered. In this way, it is possible to recover the Cu with a recovery rate of 98%.
The technique of Conventional Example 1, however, has problems as follows. The reaction product in the chemical liquid is decomposed if the recovery liquid is reserved for a long time. This is because the reactivity of the recovery liquid is extremely high, and the recovery liquid is unstable. Moreover, since each of ozone water and electrolytic ion water needs a generation device, it takes a long time to produce the recovery liquid.
In the technique of Conventional Example 2, since hydrogen peroxide in the recovery liquid forms a silicon oxide film, hydrofluoric acid is contained in the recovery liquid for etching the surface of the silicon substrate. More specifically, the formation of the silicon oxide film by hydrogen peroxide in the solution and the etching of the silicon oxide film by hydrofluoric acid in the solution are concurrently proceeded for the surface of the silicon substrate, and an impurity element attached to the surface is dissolved together with the silicon oxide film. By containing hydrochloric acid in the solution, the solubility of an impurity element into the solution is enhanced, thereby inhibiting the impurity from reattaching to the silicon wafer. Although copper (Cu) whose ionization tendency is smaller than that of silicon can be analyzed by using this method, platinum group elements (platinum (Pt), ruthenium (Ru), iridium (Ir), etc.) cannot be dissolved due to the too low HCl concentration. Therefore, it is impossible to effectively dissolve and recover the platinum group impurity elements.
An object of the present invention is to provide a platinum group impurity recovery liquid and a method for recovering a platinum group impurity, which can recover a platinum group metal contaminant on a surface of a silicon substrate or in a film thereon at a high recovery rate in an easy, safe, and highly accurate manner.
The present invention is related to a platinum group impurity recovery liquid for recovering a platinum group impurity on a silicon substrate surface or in a film thereon. The recovery liquid is a mixed aqueous solution containing HCl with a concentration of 10 to 25% by weight, H2O2 with a concentration of 2 to 5% by weight, and HF with a concentration of 0.01 to 2% by weight.
According to the present invention, the analysis recovery liquid for a platinum group impurity element comprises a mixed aqueous solution containing HCl, H2O2, and HF. HCl and H2O2 are contained so as to produce hypochlorous acid (ClOxe2x80x94). Also, since HCl is contained in the recovery liquid with a concentration of 10% by weight or higher, ClOxe2x80x94 is produced in a large amount. Such ClOxe2x80x94 produced in a large amount can dissolve a platinum group element. In the case where a natural oxide film or a thermal oxide film is formed on the surface of the silicon substrate or a film thereon to be analyzed, a platinum group element can be dissolved by ClOxe2x80x94 while etching the silicon oxide film with HF contained in the recovery liquid. As a result, a contaminant such as Pt can be inhibited from attaching to the surface of the silicon substrate. The platinum group elements refer to six elements, i.e., ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt), among elements belonging to the group VIII of the periodic table.
Another platinum group impurity recovery liquid according to the present invention is a mixed aqueous solution containing H2SO4 with a concentration of 65 to 82% by weight, H2O2 with a concentration of 4 to 16% by weight, and HF with a concentration of 0.01 to 2% by weight.
According to the present invention, a platinum group is dissolved using H2SO4 contained in the recovery liquid, and the dissolving of the platinum group can be facilitated by H2O2. Moreover, by containing a small amount of HF in the recovery liquid, even in the case where an oxide film such as a natural oxide film or a thermal oxide film is formed, a platinum group impurity element can be dissolved by H2SO4 and H2O2 while etching the silicon oxide film with HF. Therefore, a contaminant such as Pt can be inhibited from attaching to the surface of the silicon substrate.
A method for recovering a platinum group impurity according to the present invention comprises the steps of: making a droplet of the platinum group impurity recovery liquid according to claim 1 or 2 contact a silicon substrate surface or a film thereon so as to scan the silicon substrate surface with the droplet; dissolving the platinum group impurity into the droplet; and recovering the platinum group impurity.
According to the present invention, since the recovery liquid can dissolve a platinum group impurity element, when the recovery liquid is dripped onto the surface of the silicon substrate or the film thereon and the surface of the silicon substrate or the film thereon is scanned with the droplet of the recovery liquid, a platinum group impurity element attached to the surface of the silicon substrate and/or a platinum group impurity element mixed in the surface of the silicon substrate, and a platinum group impurity element attached to the surface of the film on the silicon substrate and/or a platinum group impurity element which exists in the film can be dissolved into the recovery liquid. Therefore, it is only necessary to scan the surface of the substrate or the surface of the film by using a recovery liquid which is an aqueous solution in which commercially available hydrochloric acid or concentrated sulfuric acid, hydrogen peroxide solution, and hydrofluoric acid are mixed. Since there is no need to heat the recovery liquid or the substrate, the recovery operation is easily performed. In addition, since the recovery liquid is not splashed, a platinum group impurity element can be safely dissolved into the recovery liquid, thereby making it possible to efficiently recover the platinum group impurity.
After decomposing the silicon substrate surface or the film thereon by a hydrofluoric acid vapor, the platinum group impurity recovery liquid may be used. Accordingly, the decomposition rate of the surface of the substrate or that of the film formed on the substrate can be increased.
Another method for recovering a platinum group impurity according to the present invention comprises the steps of: dripping a droplet of the platinum group impurity recovery liquid according to claim 1 or 2 onto a silicon substrate surface or a film thereon; and recovering the recovery liquid after the recovery liquid is left standing for a predetermined period of time, wherein an operation of said dropping and recovering are repeated one or more times.
According to the present invention, the recovery liquid is dripped and left standing for a predetermined period of time, thereby etching the silicon substrate or the film thereon in the depth direction at a constant rate. Therefore, by performing one or more repetitions of the operation of recovering the recovery liquid, impurities of a platinum group element in the depth direction of the substrate or the film thereon are recovered, and the concentration distribution thereof can be obtained. The film on the silicon substrate may be a silicon oxide film formed by thermal oxidation, a silicon oxide film deposited by chemical vapor deposition method, or a natural oxide film.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.